The physical properties of laser light differ fundamentally from those of conventional light sources. Laser light is coherent and can be produced as a light beam having a small, even if finite aperture angle. This narrow beam concentration is particularly advantageous for illumination and imaging purposes, since the wave fronts of the laser light approach the ideal of plane waves. They can be transformed into spherical wave fronts and can be utilized for highly resolving, diffraction-limiting focusing.
One drawback of the laser beam is its Gaussian character, which is determined by the manner in which light is generated in the resonator. The intensity distribution of the light transversely to the beam has the shape of a Gaussian bell curve. This means that the intensity is at a maximum in the middle of the beam, and it then drops off exponentially toward the edges.
This is a drawback in image processing and projection technologies which require illuminating flat photomasks. However, it is also a drawback in interferometry, where one desires a most uniform possible illumination of the lighted surface. Such uniformity is not provided when working with a Gaussian intensity profile. In material processing as well, such as in medical applications involving heating of tissue, or in laser welding, uniform heating is required over the entire width of the laser beam or of the illuminated surface. Such uniform heating cannot be attained when working with a Gaussian shaped illumination and, thus, for instance, a Gaussian energy deposition. For that reason, the cross-sectional profile of the light beam should be as rectangular as possible for the areas of application mentioned. The spatial intensity profile should be homogeneous, i.e., more or less constant over a certain width. To effect this, in practice, the beam is widened and one then works only with the more or less homogeneous inner beam region with the outer region being masked out. However, this can lead to significant intensity losses.
Since the actual laser system, the optical amplification medium in the resonator, is not readily accessible to the user, the forming of the beam into a rectangular profile must take place outside of the laser. For this purpose, optical filters, so-called “bull's eye” filters are known, which attenuate the laser beam more vigorously in the middle than at the edges, thereby flattening the bell shape of the beam profile to a virtually rectangular profile. For the most part, these filters are made of a transparent plate, e.g., a glass plate, upon which a more or less reflective coating, for example a metal, is coated by vapor deposition. The desired beam profile is produced by properly selecting the locally dependent optical density, i.e., the local transmission and reflection properties. These filters are static and, therefore, can only be used for a specific laser having a fixed, known intensity profile. When the laser changes its profile, e.g., due to fluctuations or manifestations of aging, the filters undesirably alter the shape of the profile, since they are no longer adapted to the laser data. Another disadvantage associated with reflecting filters of this kind is that unevenly reflected laser light has a reactive effect on the laser and can degrade its stability. Moreover, in place of reflecting filters, it is also generally known to use holographic filters for forming beams. See, for example, I. Gur et al.: Diffraction limited domain flat-top generator; Opt. Communications 145, 237 (1998), incorporated herein by reference. These filters are also static and are not responsive to time-related changes in the laser beam profile. Also problematic is the fact that the rectangular profile is only produced in the imaging plane of the holographic element.